Invasive versus introduced

I’ve focused a good deal of blog bandwidth of late on nonnative species in Texas, including blackbucks, aoudads, and various types of deer. Then I expanded my focus beyond Texas so as to include the now ubiquitous house finch, a bird native only in the southwestern third of North America. In each case—as throughout my years of blogging—I have religiously defined these species as “introduced” rather than “invasive” in much the same way that I define European starlings and house sparrows as “introduced” rather than “invasive”.

I define “introduced” and “invasive” a little differently than you do, I think. I don’t find the two terms mutually exclusive. Introduced simply means to me that the species is non-native, and that it has been, whether intentionally or not, introduced by humans to an environment where it didn’t previously occur and would likely never have reached on its own. Invasive in my mind refers to the nature of what a species does when it finds itself in a new place — some may sit docilely where they were put and never spread very far (many garden plants and flowers fall into this category), but some others love their new location so well that they go crazy and start spreading out from where they were put. Some introduced species may not necessarily be invasive, while some invasive species could very well be native (especially true in disturbed areas, since it’s harder for anything — introduced or otherwise — to gain a foothold in an area of stable habitat; unfortunately, our human activities mean that few habitats are truly undisturbed anymore).

As I explained in my response to that comment, I agree with her use and definitions of those terms. My tendency to mark a clear delineation between the two is intentional and strategic. By way of explaining that answer, let me introduce you to some truly invasive species.

Great-tailed grackles (Quiscalus mexicanus). Their northernmost range stopped in far South Texas as of 1900. Since then, however, they have exploded northward and westward such that now they occur year-round throughout Texas and north through Oklahoma to Kansas, and west through Colorado, Utah, Nevada and California—all the way to the Pacific Coast. Smaller migratory breeding populations stretch into Missouri and Nebraska.

Great-tailed grackles continue to expand their range in North America and southward into the northern parts of South America. This expansion is invasive as they are endemic to Mexico south to Columbia. Unlike species introduced by people, whether intentionally or otherwise, these birds have expanded their range naturally into areas where they now compete with—and in many cases out-compete—indigenous fauna.

Yet because their presence is natural, even though it is invasive, they are tolerated—even celebrated—by the same nature purists who decry and bemoan European starlings and house sparrows, calling these latter two species invasive, and who enjoy house finches east of the Rocky Mountains as though they belong there.

But let’s not stop with great-tailed grackles. Instead, let’s look at an even more invasive species of bird, one that has invaded most of the planet and is responsible for one of the fastest and largest territorial expansions of any avian species: the cattle egret (Bubulcus ibis).

Native to eastern Africa, these birds first arrived in South America in the late 1800s. They flew across the Atlantic Ocean to get there. The first confirmed cattle egret was documented in North America—in Florida—around 1940. By 1996 they had occupied the entire continental United States and southern Canada.

Their global invasion carried them to islands far and wide, not to mention all of Eurasia. In fact, they have established themselves nearly everywhere except in extreme or unwelcoming environments, such as mountain ranges, polar regions, deserts and boreal forests. Aside from those places where they can’t survive, they seem destined to occupy the entire planet, a feat they are accomplishing in short order.

Interestingly, their range is still expanding where they have invaded and their dispersal from Africa is ongoing. It might be said that as truly invasive species are concerned, cattle egrets are second only to humans.

Which brings me back to that delineation I often stick to between invasive and introduced…

The reason I make such a case for identifying European starlings as introduced as opposed to invasive is clear: nature purists like to call things invasive because it alienates the species and washes away all human responsibility. A good example is the rosy wolfsnail (a.k.a. cannibal snail; Euglandina rosea). This predatory mollusk is among the top ten most destructive nonnative species around the globe. Native to Texas, I rather enjoy watching them, but we humans intentionally introduced them all over the planet in hopes of having them control the East African land snail (a.k.a. giant African land snail; Achatina fulica), a destructive species we also had introduced all over the world.

Unfortunately for native snail species in all those places, the rosy wolfsnail not only went after the African snail, but they also went after all the native snails. This resulted in the extinction of many native snails throughout the world. It also resulted in the wolfsnail being named a major invasive species. But it’s not an invasive species! We introduced them intentionally and with forethought, but now calling them invasive makes us feel better about hating them for all the damage they’ve done.

And thus comes the crux of the matter: both invasive and introduced species do what they do because they evolved naturally to do those things, whether it be killing snails or out-competing native birds. In the case of truly invasive species, they move into nonnative ranges of their own volition and fight to claim new territory and resources; in the case of introduced species, we put them in nonnative ranges and then act shocked when they run amok—at which point we start calling them invasive so we can hate them and destroy them without admitting we are the problem, not the species we’re so angry at.

So I reiterate what I said to Seabrooke: I wholeheartedly agree with her understanding and use of “invasive” and “introduced”. However, I will continue to draw a line in the sand between the two terms. There is a massive difference between the two in light of how people tend to use them and what people justify by way of them; so few nature purists realize the hypocrisy or cognitive dissonance they express by picking and choosing which invasive species should be hated and which introduced species should be considered invasive; calling something invasive stigmatizes a species in the public’s eye and misinforms lay people as to who precisely is responsible for the damage that species is doing; and very few seem to recognize or appreciate the laughable irony of having humans call any species invasive because it’s harming native flora and fauna.

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One thought on “Invasive versus introduced”

Although I’m going to stick by my guns 🙂 I respect the arguement you’ve presented here, Jason. I do agree that the sentiments aroused are different when one or the other term is used. Truthfully, we should be calling the Rosy Wolfsnail “introduced and invasive”, and not one or the other, but I guess that’s a bit of a mouthful. 😉

I have no experience with either Great-tailed Grackles or Cattle Egrets, so I can’t comment on those situations, but a bird with a parallel story up here is the Brown-headed Cowbird. Historically it was a species of the great plains where it would follow the roaming bison herds, picking insects from the grass and dirt that the huge animals disturbed. Because the bison were nomadic and never stayed in one place long enough the cowbirds evolved to lay their eggs in other species’ nests.

Then came the early settlers, clearing the forests for agriculture and bringing with them herd animals such as sheep and cattle. And voila! Instant cowbird habitat! Suddenly, all of the east was their oyster. As a species, Brown-headed Cowbirds are tens of thousands of years old and had never left the plains in all that time, but within the span of a century or two – an eyeblink, evolutionarily – they had expanded their range to include much of the east.

If these early pioneers hadn’t arrived and altered the habitat, Brown-headed Cowbirds would remain a prairie species. By your definitions, they’re an invasive species, having spread out naturally of their own accord, but humans are the ultimate cause of this abrupt change in distribution.

I suspect this is probably true of both Great-tailed Grackles and Cattle Egrets, too. That egret in the 1800s was probably not the first one in the history of the species to attempt the ocean crossing, but it’s likely that he’s the first one to find conditions to his liking when he arrived, because of changes humans had made to the environment. We’re still to blame for their spread, even if it wasn’t our hand that deliberately placed them in their new location.

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My name is Jason M Hogle. I'm a friend, son, brother, writer, photographer, scientist, naturalist, mathematician, conservationist, poet, technology nut, and many other things. I consider myself a Renaissance man. I have a passion for learning. I also have a passion for living. What you see here is who I am, from my triumphs to my tribulations and everything that fills the space betwixt the two. If you're still curious, read more about me.

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